Multiple beam charged particle optical system

ABSTRACT

The invention relates to a multiple beam charged particle optical system, comprising an electrostatic lens structure with at least one electrode, provided with apertures, wherein the effective size of a lens field effected by said electrode at a said aperture is made ultimately small. The system may comprise a diverging charged particle beam part, in which the lens structure is included. The physical dimension of the lens is made ultimately small, in particular smaller than one mm, more in particular less than a few tens of microns. In further elaboration, a lens is combined with a current limiting aperture, aligned such relative to a lens of said structure, that a virtual aperture effected by said current limiting aperture in said lens is situated in an optimum position with respect to minimizing aberrations total.

CROSS-REFERENCE TO RELATED APPLICATION

The present patent application is a Divisional of non-provisionalApplication No. 13/050,875, filed Mar. 17, 2011, which has issued asU.S. Pat. No. 8,188,450, on Mar. 17, 2011, which is a Divisional ofnon-provisional Application No. 11/880,872, filed Jul. 23, 2007, whichapplication issued on Mar. 13, 2012 as U.S. Pat. No. 8,134,135, whichapplication claimed the benefit of Provisional Application No.60/833,394, filed Jul. 25, 2006.

BACKGROUND OF THE INVENTION

The present invention relates to a charged particle beam system such asan electron beam exposure system, scanning and non-scanning electronmicroscopes, and the like.

1. Field of the Invention

The current invention presents a microlens array for generating aplurality of focused beam lets or focusing beams with different incidentangles in a charged particle beam exposure or imaging apparatus withzero field curvature, minimized geometrical aberrations such as coma andastigmatism, comprising a current limiting aperture, generating aplurality of charged particle beamlets, a lens array aligned with thecurrent limiting aperture, for focusing all the beamlets into a flatimage plane. It solves the problem of how to generate a plurality offocussed beamlets with a minimum of aberrations in a diverging beamsystem, such as in multi-beamlet inspection systems, in particulardesired at a limited source to target distance such as in a multibeamlet, maskless lithography system.

2. Description of Related Art

The progress in microelectronics, microfabrication and material sciencedemand an ever-increasing spatial resolution and throughput in chargedparticle beam lithography and inspection. Conventional single beamsystems suffer from coulomb blur and low throughput. Several chargedparticle beam systems featuring multi-beam, multi-column and/ormulti-source are under development to solve the contradictoryrequirements. To increase the throughput of charged particle beamsystems and avoid coulomb blur however, a large exposure field isdesired, which requires the use of not only axial, but also off-axialbeam of the diverging beam emitted from a charged particle emittingsource. By introducing a lens array that can generate a plurality offocused beams, i.e. can focus beams with different incident anglesCoulomb blur can be avoided as disclosed by Applicants earlier patentpublication cited hereafter. At the same time, a tight control of theoff-axial aberrations is desired.

In JP60011225, JP60039828 and J. Vac. Sci. Technol. B 4(5),September/October 1986, an electron matrix lens with reduced aberrationsis disclosed where the centre of a current limiting aperture is shiftedfrom the optical axis of a lens associated with said aperture, to anoptimum position. The position of the current limiting aperture, whichis fabricated in a separate plate, is chosen such that a virtualaperture, which is symmetrical along the optical axis, is in a positioncausing the total aberration for off-axial lenses to be minimal. Patentpublication JP60042825 discloses a correction means for the fieldcurvature for each lens, by changing the focus using a correction lensmatrix. However, the astigmatism becomes dominant with increasingincident angle and eventually restricts the maximum incident angle toless than 30mrad, thus the throughput of the system is limited. Thethroughput of the system is also restricted due to a small fillingfactor of the lens that is allowed.

In WO2004/081910 in the name of Applicant, a lens array forming aplurality of focused beamlets from a diverging broad beam is disclosed.FIG. 10A thereof discloses a schematic of an example of the lens array,but does not indicate what the position of the electrode must be withrespect to the beam. In order to reduce aberrations, the lens is, in aparticular embodiment, concaved with respect to the source so that theoff-axial beamlets can pass the lens along the optical axis. In thissolution, the curvature of the lens plate leads to undesired engineeringchallenges. Also, drawbacks include that the image plane for all thebeamlets is present in a concaved surface with respect to the source.Further, a difficulty exists in the alignment between the lens array anda “spatial filter”, in fact the current limiting aperture structure,which in this publication is taught to be on a planar surface.

BRIEF SUMMARY OF THE INVENTION

It is a particular objective of present invention to realise a microlens structure as is conceptually disclosed in the latter said WOpublication.

It is a further objective of the invention to realise an alternative, inparticular improved lens structure with respect to the lens structure asdisclosed in the first said JP publications.

It is also an objective of the present invention to create a flat imageplane for a micro lens positioned in a diverging beam.

A further objective of the invention is to minimize the imageaberrations for the realised lens array.

Another objective of this invention is to improve the resolution ofcharged particle beam systems departing from a source with at least onediverging charged particle beam.

Again another objective of this invention is to improve the throughputof such charged particle beam systems.

Yet another object of this invention is to control the uniformity of thebeams.

So as to meet at least part of these objectives, the present inventionrelates to charged particle optical system as defined in claim 1. Withthis measure according to the invention, the dimension of the effectiveelectric field height is reduced by a factor of thousand relative to theprior art micro lens structures, resulting in strongly reduced chance ofeffectively having a beamlet passing through a lens part with stronglysub-optimal conditions with respect to image aberrations. Though itcould perhaps be possible to optimise the measure according to theinvention, such will be performed by dimensioning the lens with changeswithin order of the new lens and effective field size, i.e. will remainwithin the order of ultimate smallness as claimed, totally different ineffect and basic principle than the measure that has been taken with thepresent invention.

The invention, in a further elaboration thereof also relates to anapparatus for generating a plurality of focused charged particlebeamlets or focusing beams with different incident angles in chargedparticle beam systems, comprising:

a) a current limiting aperture array located either before or after thelens array, to split up the diverging charged particle beam into aplurality of charged particle beamlets;

b) a lens array comprising a plurality of lenses to focus the beamletswith different incident angles into a plane;

c) the above said current limiting aperture being aligned with the lensin the lens array for a beamlet with a particular incident angle suchthat the virtual aperture is symmetrical along the optical axis and islocated optimised in the lens with respect to resulting aberrations,e.g. by a, though not necessarily, centred location.

In this way, it is possible to improve the throughput by maximallyutilizing the diverging current emitted from a source with sufficientbeamlets per area on the surface of target. Furthermore, havingsufficient large current per beamlet is in accordance with an insightand purpose underlying the present invention possible by minimizing theaberration for each lens. Having minimised the total of aberrationsallows for increasing the opening angle of a beamlet, which favourablyincreases current. Also, homogeneity of the beamlets in terms ofaberrations and current can be controlled by adjusting the parameter ofthe lenses in the lens array.

In an embodiment, the current limiting aperture is aligned with eachlens in the lens array. The current limiting aperture is according toinvention preferably fabricated on the same plate as either the first orthe last lens electrode, but can be on a separate plate. The currentlimiting aperture is positioned in a field free region, while thediameter, more in general the size, largeness or magnitude of usesurface area, of the current limiting aperture may change forhomogeneity of beam currents, in particular as a function of itsdistance to the centre of the lens array.

In a further elaboration, the lens array comprises of two planarelectrodes, which are in a separation of less than a few tens ofmicrons. The two electrodes are aligned with respect to each other. Thebore diameters, in general the bore size, in the two electrodes are thesame, and smaller than the thickness of the electrodes to limit the lensfield from penetrating deep into the lens holes, alternatively denotedlens apertures. The lens size, in the specific embodiment of acylindrical opening, the diameter, increases for off-axial lenses forfield curvature correction.

In another embodiment, the lens array comprises of a single planarelectrode, with at least two, preferably three macro-electrodes facingthe lens holes. The first electrode has the same or higher potential asthe aperture lens electrode, while the second electrode has a higherpotential than the first electrode, and the third electrode has a lowerpotential than the second electrode. The diameter of the said aperturelens is smaller than the thickness of the said lens electrode to limitthe lens field from penetrating deep in to the lens holes. In a furtherelaboration, having realised that the field penetrating from themacro-electrodes forms the aperture lens effect in the aperture lensholes, the strength of the off-axial aperture lens in the aperture lensarray is made weaker by having the field in front of the said aperturelens weaker than that of the central lens. In this way, by using thelarger focal length at the off axial lenses, the field curvature iscorrected, i.e. the image plane is brought into a planar surface. Thelens diameter preferably increases for off-axial lenses for fieldcurvature correction.

In yet a further embodiment, the lens array comprises three planarelectrodes and the opening angle limiting aperture is made on a separateplate. The three electrodes are aligned in such a way that the centre ofthe beam with a specific incident angle passes through the centre ofeach electrode. The size of the lens hole cross section, e.g. expressedby a diameter, is preferably made larger for off-axial lenses for fieldcurvature correction.

In yet a further elaboration, irrespective of any specific embodiment,the lens holes are made elliptical for correction of astigmatism. It isin this respect to be noted that in principle, most if not all of thefeatures described as embodiments or not in this document, may becombined.

For each type of lens array, different microfabrication process flowshave been developed, aiming at high productivity and better lensperformance, the details of which are not described here. Further, thepresent invention may alternatively also be defined as in the following,paragraphed definitions.

A micro-lens array with limited lens field: in case of a two-electrodelens array, the separation between two electrodes is less than a fewtens of microns, and the lens bore diameter, in case of an ellipticalshape, the smallest diameter, is smaller than the thickness of the lenselectrode; in case of aperture lens array, the lens diameter is smallerthan the thickness of the electrodes. In this way, the third orderaberrations, especially coma and astigmatism, will be minimized foroff-axial beamlets.

The first order field curvature due to a longer objective distance foroff-axial beamlets is compensated by increasing the radius of theoff-axial lens holes, so that each lens of the lens array focuses abeamlet at the same image plane.

For the aperture lens array, alternatively, the field curvature can becorrected by adding three macro electrodes facing the lens aperture,with the potential of the first macro electrode the same or higher asthat of the aperture lens array. This configuration leads to a curvedequal-potential plane in front of the aperture lens array. The curvatureof the equal-potential plane leads to a smaller aperture lens strengthfor the off-axial lenses than that of the central lens, and in this way,the field curvature can be corrected.

The apparatus mentioned above can be either an aperture lens withshifted current limiting apertures, a two electrode microlens array madeof a SOI wafer or a two electrode microlens array by bonding of twowafers.

Alternatively, a three-electrode lens array can be used for generating aplurality of beamlets or focusing beams with different incident angles,where the lens electrodes are skewed in such a way that the centre ofthe beam passes through the center of each electrode. The fieldcurvature may be corrected by increasing lens radius for off-axial lensin the lens array. Elliptical lens holes may be used to correctastigmatism. In this case, the current limiting aperture is made on aseparate plate.

The current limiting apertures said in above definitions and the firstor last lens electrode are made of one piece of wafer, the alignment isdone with optical lithography. The current limiting apertures mentionedin the above definitions is in a field free region by limiting the lensbore diameter smaller than the electrode thickness. The diameter of thecurrent limiting aperture may change tor homogeneity of beamletcurrents.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will, by way of example be further elucidated in thefollowing embodiments of a charged particle optical system according tothe current invention, in which;

FIG. 1 is a schematic illustration of a charged particle or light opticbeam passing a lens system with a lens and a beam current limitingopening;

FIG. 2 illustrates a two-electrode micro lens, here created by bondingof two wafers, and showing a shifted apertures;

FIG. 3 represents an alternative to the embodiment of FIG. 2, by havingan electrode produced of an SOI wafer;

FIG. 4 schematically illustrates two equivalent integrated aperturelenses with shifted current limiting aperture;

FIGS. 5 and 6 illustrate embodiments of an aperture lens array combinedwith three macro lenses, FIG. 6 thereby illustrating the working ofequipotential lines, and the requirement to consider the same at thelocation of a lens array;

FIG. 7 illustrates a micro-einzel-lens with shifted electrodes;

FIG. 8 illustrates a micro-einzel-lens array with field curvaturecorrection means;

FIG. 9 illustrates the difference in size and configuration between aprior art lens and a lens in accordance with the invention.

In the figures, corresponding structural features, i.e. at leastfunctionally, are referred to by identical reference numbers.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an alignment of a current limiting aperture CLA, normallypart of an array of such apertures CLA and included in an aperture plateAP, and a lens 2. The current limiting aperture CLA of the lens is insuch a position, that for a beam 3, such as a charged particle beam,with a certain incident angle α, the virtual aperture VA is located in acentre part of the lens 2, and symmetrical along the optical axis OA ofthe lens. Here, the middle, i.e. centre of the lens means the geometrycentre of the lens. In case of a two electrode lens, THE middle of thelens is at the middle of two electrodes and the IN case of threeelectrode lens, the middle of the lens is at the middle of the centralelectrode, and in case of an aperture lens, the middle of the lens is atthe end plane of the aperture. The image further shows an image planeIP.

In FIG. 2 and FIG. 3, two examples of two-electrode lens arrays arepresented in the form of lens structures 5, 6, in short also denotedlenses. Compared to conventional charged particle lenses, where thefilling factor is typically 10%, the filling factor for the presenttwo-electrode lenses can be 85% and even higher, without significantincrease in coma, astigmatism and field curvature. These lens structurescome with aperture plates AP, which are here integrated with oneelectrode, thereby forming a so-called integrated aperture lens array.In the lens structures, the two electrodes are aligned with each other.The lenses show cylindrical holes H1, of which the diameter dl issmaller than the thickness De of the first electrode. The effect of suchmeasure being that the effective electric field Ef, illustrated in FIG.9, of the lens is prevented from penetrating deep into the lens holesH1. The current limiting aperture CLA is fabricated on the first lenselectrode, and is positioned in a field free region. The radius ofoff-axial lens hole H1 increases in order to correct the first orderfield curvature, and, surprisingly, the radius variation appears to havea large effect on the lens strength, more than enough to correct thefirst order field curvature. By doing so, the sizes of the images formedby the lens array are equal to each other, and without inducing furtheraberrations.

In FIG. 4, a schematic of an aperture lens is shown, where the apertureplate AP with current limiting aperture CLA and the lens holes H1 aremade of a piece of wafer. The current limiting aperture CLA is in such aposition that the virtual aperture VA is at the end plane of theelectrode E and symmetric along the optical axis. The lens bore diameterdl is smaller than the thickness De of the electrode in order to limit apertaining lens field Ef from penetrating deep into the lens holes H1.The right hand side of FIG. 4 indicates an equivalent version whereinthe sequence of lens opening or electrode and the current limitingaperture array is inversed with respect to the direction of an incidentbeamlet.

In FIG. 5, an example of an aperture lens array, here an integratedaperture lens array IAL is shown, with three macro electrodes ME1, ME2and ME2 facing the aperture lens, i.e. all of the beams that are passedthrough the aperture lens array, pass the set of macro-electrodes ME1,ME2 and ME3, in the central part thereof. The first electrode ME1 is ata same or higher potential V1 as a potential V0 at the aperture lensIAL, i.e. V0=<V1, while the second electrode ME2 is at a higherpotential V2 than V1, i.e. V2>V1. The third large electrode ME3 is at apotential V2 smaller than that V2 of ME2, i.e. V3=<V2. The figureindicates equi-potential lines EPL for this alternative embodiment.Aperture lenses are here formed at the lower end of a bore or lens holeH1. It may be clear form the illustration that with the equipotentiallines locally being closer to one another and to the aperture lens AL,the central aperture lens, or lenses as the case may be in largerembodiments, is respectively are stronger than that of relativelyoff-axial aperture lenses. Thus the central aperture lens is strongerthan the off-axial lenses, which phenomenon is here used for fieldcurvature correction. In fact an increased focal length at the off axislens holes causes, the normally curved image plane to become, at leastvirtually, flat. Alternatively and additionally when desired, the fieldcurvature is also corrected using increasing lens size, here, withcircularly shaped apertures, with increasing radius dl, for off-axialaperture lenses AL. The images formed by the present aperture lens arrayIAL are projected onto a flat image plane FIP.

FIG. 6 provides a functionally equivalent example of a largely inversedarrangement, wherein the current limiting apertures are located downwardfrom the macro-lenses ME1-ME3 in view of the main direction of thebeamlets 3, and with appropriately adapted voltages. Where the exampleof FIG. 5 provides for a collimating effect of the macro lenses on thebeamlets, in the present example the arrangement may be set into a zerostrength mode and a non-zero strength mode of operation. The non-zeromode is here illustrated and provides for a corresponding focusingeffect as is the case in the FIG. 5 arrangement, while in thenon-depicted zero strength mode, the field is solely applied forgenerating a plurality of focused beams with to function with fieldcurvature correction. Because the aberrations are proportional to therefraction power of the lens, thus when the macro lens is operating atzero strength mode, the aberrations, in particularly, field curvatureand chromatic deflection error, are small, i.e. zero strength macro lensdoes not introduce extra aberrations.

FIG. 6 furthermore illustrates the effect of equipotential lines in asaid three macro-lens lens structure in slight more detail, and alsoshows the electric field effect of the macro lenses ME1-ME3. The figurein particular illustrates how the equipotential lines are most close toone another in the most centered lens holes, and somewhat further awayfrom one another at the off-axial lens holes.

FIG. 7 shows a 3-electrode lens, comprising a current limiting apertureCLA made in a separate aperture plate AP, and 3 electrodes Es1-Es3. Thecurrent limiting aperture CLA is aligned to the three electrodes Es1-Es3in such way that the virtual aperture VA is in the centre part of themiddle electrode and symmetrical along the optical axis of each lens.The three electrodes E1-E3 are skewed in such a way that the centre ofthe beam passes through the center of each electrode hole H1.

In FIG. 8, the 3-electrode lens array is shown with lens radiusincreasing for off-axial lenses, so as to correct field curvature. Theimages of each lens are by this lens structure projected onto a flatimage plane. Additionally to this measure, elliptical lens holes H1 mayaccording to the invention be used to correct for leftover astigmatism.

FIG. 9 provides an illustration of difference in size and configurationof a prior art lens structure, e.g. as in the earlier cited J. Vac. Sci.Technol. B 4(5), September/October 1986, in the upper figure part,relative to that of the structure according to the invention,represented on scale in the encircled figure part, and provided as anexploded view in the lower figure part. Also from the latter is evidentthat the largest dimension dl in a cross section of a lens hole HL isequivalent and preferably smaller than the thickness of a lenselectrode, while the lens field is confined to within the thickness ofthe electrode.

Apart from the concepts and all pertaining details as described in thepreceding, the invention relates to all features as defined in thefollowing set of claims as well as to all details in the annexed figuresas may directly and unambiguously be derived by one skilled in the art.In the following set of claims, rather than fixating the meaning of apreceding term, any reference numbers corresponding to structures in thefigures are for reason of support at reading the claim, included solelyfor indicating an exemplary meaning of a preceding term and are thusincluded between brackets.

The invention claimed is:
 1. A charged particle lithography system fortransferring a pattern onto the surface of a target, such as a wafer,comprising: a charged particle source adapted for generating a divergingcharged particle beam; a converging means for refracting said divergingcharged particle beam, the converging means comprising an electrode; andan aperture array element comprising a plurality of apertures; whereinan electric field between the electrode and the aperture array ispresent.
 2. System according to claim 1 wherein the converging means isincluded as a collimator for projecting the curved image plane of thediverging beam onto a flat image plane.
 3. System according to claim 2,wherein the converging means comprises three electrodes forming anEinzel lens.
 4. System according to claim 3, wherein the first electrodeis energized at a relatively higher electric potential with respect tothe second electrode and the third electrode is energized at arelatively lower electric potential with respect to the secondelectrode.
 5. System according to claim 4, wherein the apertures of theaperture array have a narrowest portion recessed below an outer surfaceof the aperture array.
 6. System according to claim 5, wherein saidnarrowest portions of the apertures are recessed to a field free region.7. System according to claim 6, wherein the apertures in the aperturearray are included as current limiting apertures.
 8. System according toclaim 7, wherein the converging means is positioned upstream from theaperture array element with regard to the charged particle source andwherein the apertures are recessed below the surface of the aperturearray facing the charged particle source.
 9. System according to claim8, wherein the charged particle source is positioned upstream from theconverging means with regard to the charged particle source and theconverging means is positioned upstream from the aperture array element.10. System according to claim 9, wherein the electric field is adecelerating electric field.
 11. System according to claim 10, whereinthe electric field is created by energizing the final electrode of theconverging means at a relatively higher electric potential with respectto the aperture array element.